a. Field of the Invention
Broadly speaking, this invention relates to oscillators. More particularly, in a preferred embodiment, this invention relates to methods and apparatus for synchronizing an oscillator with a reference source of known frequency.
B. Discussion of the Prior Art
As is well known, a substantial portion of all long distance telecommunication is satisfied by microwave radio systems operating in the 4 and 6 GHz common carrier bands. Such systems typically employ frequency modulation and no excessive demands are placed on the frequency stability of the oscillators employed, other than they must, of course, satisfy the requirements of the Federal Communications Commission.
Recently, a proposal has been made that such existing FM systems be converted to single-sideband operation which will effectively double system capacity with virtually no increase in plant. However, single-sideband operation demands far greater frequency stability for the microwave oscillators employed than does frequency modulation. Of course, it would be possible to employ a stable reference frequency source, operating at a lower frequency, and then multiply this source up in frequency to the desired output frequency in the 4 GHz or 6 GHz common carrier band. However, this unsophisticated approach would be very expensive and, in view of the large number of different channel assignments commonly employed in such microwave systems, would require a series of individually designed circuits for each microwave system.